Apparatus for removing material, use of gas inclusions in an abrasive liquid and process for grinding and/or polishing surfaces

ABSTRACT

The invention relates to an apparatus for removing material from a surface of a workpiece during grinding and/or polishing of the surface by means of abrasive particles which are delivered by a liquid. The apparatus comprises a device for setting the gas content in the liquid, in particular for adding gas, in particular air, to the liquid.

The invention relates to an apparatus for removing material, to the useof gas inclusions in an abrasive liquid and to a process for removingmaterial during grinding and/or polishing of surfaces of workpieces, inaccordance with the features of the independent patent claims.

It is known to grind or polish surfaces using abrasive particles whichare delivered in a liquid. This has the advantage that there is no needfor a grinding tool.

The material-removal rate from the surface depends firstly on the sizeof the abrasive particles in the jet. Secondly, it depends on the energyand therefore the velocity of the particles at the instant that theystrike the surface. The material-removal rate is to be understood asmeaning the amount of material removed per unit time.

U.S. Pat. No. 5,700,181 has disclosed, for example, a process forgrinding and a nozzle for accelerating the abrasive liquid. The abrasiveliquid is delivered into the nozzle at a high pressure and from thenozzle is accelerated onto the surface. The acceleration takes placefirstly in the nozzle and secondly in the gap between the nozzle and theworkpiece which is to be processed.

A similar process is known from EP 1 409 199. A tool with a wide outletis used instead of a nozzle. The acceleration of the abrasive particlestakes place in the gap between the outlet and the surface.

These known processes have the drawback that processes which removelarge amounts of material, in particular grinding, have to use coarserabrasive particles than precision-grinding and polishing processes. Thecoarser particles also produce coarser grinding tracks, which then haveto be smoothed using the finer grain. Also, the transition from grindingto polishing cannot take place continuously. The abrasive has to beswitched between grinding and precision grinding/polishing.

The use of particles of different coarseness requires extremely thoroughcleaning of the equipment and the workpiece if finer grinding orpolishing is to be carried out. If coarser particles are still presentduring the precision grinding or polishing, these particles lead to deepscratches, which can render the entire workpiece unusable.

Different appliances are often used for grinding and polishing. Theseappliances can, for example, work at different pressures. This increasesthe procurement or production costs. Moreover, the workpiece has to betransported during the processing operation. This in turn increases theworking time and the production costs. Moreover, contamination canresult or the workpiece may be damaged.

The object of the present invention is to avoid the drawbacks of theknown prior art, i.e. in particular to provide an improved apparatus andan improved process for grinding and/or polishing surfaces, in which itis simple to change between grinding and polishing. In particular, thematerial-removal rate is to be set individually in the simplest possibleway.

This object is achieved by an apparatus, by the use of gas inclusions inthe abrasive liquid and by a process for removing material duringgrinding and/or polishing of surfaces in accordance with the independentpatent claims. Surprisingly, it has been found that for the same size ofabrasive grain and the same process parameters (e.g. delivery pressure),the material-removal rate rises with an increase in gas content. If theapparatus is provided with a device for setting the gas content in theliquid, therefore, it is easy to control or alter the material-removalrate. It is possible, by virtue of the present invention, to achievehigh material-removal rates with relatively small grain sizes. Setting ahigh gas content increases the material-removal rate without largerabrasive grain having to be used.

In this context, it is advantageously also possible for thematerial-removal rate and also the transition from rough grinding toprecision grinding and to polishing to be varied continuously. Also, thegrain size of the abrasive grains and the delivery pressure of theabrasive liquid can be kept the same for both rough and precisiongrinding and for polishing. The material-removal rate is controlled andif necessary altered in a simple way by setting the gas content in theliquid. It is possible to use the same delivery pressure and the samesize of abrasive grain. There is no need to change from one grain sizeto another, with the associated cleaning of the machine.

The apparatus for removing material from a surface of a workpiece duringgrinding or polishing of the surface by means of abrasive particleswhich are delivered by a liquid for this purpose preferably comprises adevice for admixing gas to the liquid.

This gas is preferably air, since air is inexpensive and does notpollute the environment. Other gases or gas mixtures, e.g. argon,helium, nitrogen, CO2 or oxygen, can also be used, however, and are notexcluded from the invention. The gas may be present in the liquid eitherin the form of gas bubbles or in dissolved or bound form.

The present invention brings the advantage that a workpiece can be bothground and polished as required by means of a single apparatus. The moregas the abrasive liquid contains, the greater the material-removal rate.Expensive steps which are also tricky in terms of the processengineering involved, such as cleaning of the workpiece or the applianceor transporting of the workpiece from one appliance to another, aretherefore eliminated.

The advantages of the present invention are particularly advantageousfor the grinding and polishing of high-precision parts, such as forexample parts used in optics or medical technology. The broadversatility of the apparatus and process allows both spherical andaspherical shapes to be accurately ground and polished.

It is advantageous if the device for admixing the gas to the liquidcomprises an arrangement for setting the quantity of gas added. In thiscase, it is advantageous if the quantity of gas supplied can be variedcontinuously. This makes it possible, for example, to increase thematerial-removal rate by increasing the quantity of gas. Reducing thegas quantity leads to less material being removed. Precision-polishedsurfaces can preferably be achieved by complete removal of the gas.

The device for admixing the gas to the liquid may, for example, bearranged in a tank for the liquid. It is also conceivable for the deviceto be arranged in a pump for delivering the liquid or in a delivery linefor the liquid. Combinations of these arrangements for more efficientand controlled addition of the gas are advantageous.

The gas may, for example, be added using a propeller in the tank.Furthermore, it is possible to provide a pressure connection to theline, the tank or the pump. Moreover, it is also possible for the gas tobe added chemically, e.g. using tablets. By way of example, a chemicalsubstance which has been pressed into the form of a tablet and whichreacts on contact with water so as to release gaseous reaction productscan be added, in a similar way to effervescent tablets. It is alsoconceivable for gas to be added electrolytically to the water or for gasto be generated electrolytically in the water. By way of example, byapplying a voltage in the abrasive liquid, it is possible to separatewater molecules in the abrasive liquid into hydrogen and oxygen.Combinations of the above possibilities as well as further possibilitieswith which the person skilled in the art will be familiar are alsoencompassed by the invention.

The gas content is controlled and set in accordance with the invention.The gas quantity (in percent by volume) is typically set to desiredvalues. The gas may either be dissolved or bound in the liquid or maytake the form of bubbles in the liquid.

It is also possible to use a regulating arrangement for regulating thegas content. In this case, the gas content in the liquid is measured onan ongoing basis and set as a function of this measurement.

If the gas is dissolved or bound, the mixing ratio can be determined onthe basis of the viscosity of the abrasive liquid. The higher the gascontent in the liquid, the lower the viscosity becomes.

If the gas is in the form of bubbles, the bubble size can be determined,for example, by an optical route. To have a favorable influence on thematerial-removal rate, it is advantageous if the gas bubbles which formare larger than the abrasive particles.

In both cases, the measurement preferably takes place between a pump forthe abrasive liquid and an outlet opening for the liquid.

The material-removal rate at the surface of the workpiece is controlledby means of the gas content in the liquid. The higher the gas contentsin the abrasive liquid the higher the material-removal rate.

The material-removal rate depends on the impact energy of the abrasiveparticles. The transition from grinding to polishing of surfaces can becontrolled directly by means of the gas content in the abrasive liquid.The lower the gas contents in the abrasive liquids, the lower thematerial-removal rate, and therefore the more the surface is polishedrather than ground. In the case of brittle surfaces, e.g. in the case ofworkpieces made from ceramic or glass, the transition from grinding topolishing may at the same time correspond to a transition from brittleto ductile removal of material from the surface (cf. for example Appl.Opt. 37, pp. 6771-6773 (1998)).

It is assumed that the material-removal rate is increased by the gascontent in the liquid because the gas inclusions can be bettercompressed on impact than the liquid itself. The abrasive particles aretherefore decelerated to a greater extent by the liquid than by the gasshortly before impact. The higher the gas content therefore, the greaterthe impact energy of the abrasive particles.

Furthermore, particles which are in a gas bubble are less stronglycooled on impact on the surface than particles which are surrounded byliquid. It is assumed that the increased temperature increases theprobability of cracks forming. This increases the material-removal rate,since during brittle removal of material the removal of the material ispreceded by the formation of a crack on the surface.

If the gas is not in the form of bubbles, but rather is dissolved in theliquid, the viscosity of the abrasive liquid drops. As a result, theabrasive liquid is accelerated to a greater extent for the same deliverypressure. This greater acceleration increases the impact energy of theabrasive particles.

Moreover, it is possible that further factors contribute to theincreased material-removal rate.

The apparatus according to the invention is preferably designed togenerate a delivery pressure of less than 100 bar, particularlypreferably of less than 50 or even less than 20 bar. If, according toone of the above preferred embodiments of the invention, a gas isadmixed with the liquid, it is possible to increase the impact energy ofthe abrasive particles. This allows the delivery pressure to be reducedfor the same material-removal rate. Consequently, energy is saved ondelivery compared to high-pressure arrangements, which has both economicand ecological benefits. A further reduction in the delivery pressurecan be achieved by corresponding, suitable arrangements for acceleratingthe abrasive liquid. By way of example, it is possible to provide anarrangement for accelerating the abrasive liquid in which the abrasiveliquid is advantageously accelerated to a velocity of over 20 m/s. As aresult, the abrasive particles acquire the required energy for materialto be removed. The increase in the energy of the particles is the squareof the increase in the velocity of the jet.

The acceleration arrangement may, for example, have one or more nozzles.

The arrangement for accelerating the abrasive liquid may also include anarrangement for setting the distance between at least one outlet openingfor the liquid and the surface. The acceleration can then be set betweenthe at least one outlet opening and the surface as a function of thecross section of a liquid feed opening and the cross section of the gapbetween the surface and the at least one outlet opening. The smaller thegap by comparison with the cross section of the liquid feed opening, thegreater the acceleration of the abrasive liquid. By way of example, thearrangement could be configured in accordance with EP 1 409 199.

The combination of mechanical measures for increasing the outletvelocity and the addition of gas to increase the impact velocity of theparticles on the surface to be processed is particularly effective atreducing the delivery pressure required. At the same time, individualsetting of the material-removal rate is possible on account of thecontrollability of the gas content. It is particularly advantageous thata better material-removal rate can be achieved for the same processparameters and the same abrasive liquid by supplying gas.

The abrasive particles preferably have a mean grain diameter of lessthan 50 (m, preferably from 1 to 10 (m. However, the grain size variesaccording to the particular application. The more finely the surface isto be polished, the finer the abrasive particles that are used.

If gas bubbles are generated, their diameters should be greater than thediameter of the abrasive particles.

The delivery liquid is preferably predominantly water. However, otherknown liquids are also within the scope of the invention. Other liquids,for example based on oil or alcohol, may also be suitable.

The use of gas inclusions in a delivery liquid for abrasive particlesfor influencing the material-removal rate during the removal of materialfrom a surface of a workpiece during grinding or polishing of thesurface is likewise within the scope of the invention. The gas ispreferably air.

During the process for removing material from a surface of a workpiecefor grinding or polishing the surface using abrasive particles which aredelivered by a liquid, the material-removal rate is influenced bysetting the gas content in the delivery liquid, in particular byadmixing a gas to the abrasive liquid. It is preferable for the gas tobe air, although other gases are not excluded from the invention.

It is preferable for gas amounting to 0 to 70% of the total volume to beadded to the abrasive liquid. The greater the quantity of gas added, thehigher the material-removal rate. No gas or only a small amount of gasis added for polishing of the surface, since a low material-removal rateis desired during polishing.

It is quite conceivable to vary the quantity and/or type of gas bubblesgenerated and/or the mixing ratio of the dissolved or bound gas withrespect to the liquid during the material-removal process. For example,it is conceivable in particular to achieve high material-removal ratesin a first step by supplying a large quantity of gas and then to reducethe quantity of gas supplied in a second step during the same processingprocess, in order thereby to reduce the material-removal rate. In thiscase, polishing takes place in a second step in which less gas or no gasis added.

In a preferred exemplary embodiment, the reduction in the gas supply iscontinuous. Consequently, the transition from grinding to polishing isnot discrete.

It is advantageous for the delivery pressure of the abrasive liquid tobe kept constant at less than 100 bar, advantageously less than 50 bar,particularly advantageously less than 20 bar, for both grinding andpolishing. This simplifies the process, since there is no need to alterthe delivery pressure of the liquid if the material-removal rate is tobe varied.

The invention is explained below on the basis of exemplary embodimentsand drawings, in which:

FIG. 1 diagrammatically depicts an exemplary embodiment of the apparatusaccording to the invention,

FIG. 2 diagrammatically depicts an alternative embodiment of theapparatus according to the invention, and

FIG. 3 diagrammatically depicts a further alternative embodiment of theapparatus according to the invention.

FIG. 1 diagrammatically depicts a first exemplary embodiment of theinvention. A workpiece 2 is located on a holder 10 in a closed vessel11. The surface S which is to be ground and polished is on the top sideof the workpiece 2.

The holder 10 can be displaced along a longitudinal axis and rotatedabout a vertical axis. The movement of the holder 10 is indicated by thearrows B.

A liquid 3, in this example water, is located in a liquid tank 12.Abrasive particles 5 have been added to the water 3 in a concentrationof 10% by weight. Abrasive particles 5 of silicon carbide with a grainsize of 7 (m diameter are used. The abrasive liquid 3 is delivered by apump 7 through the delivery line 14 in the direction indicated by thearrows 6. The delivery pressure is 12 bar.

The removal of material takes place at room temperature, i.e. at approx.21° C. The mechanical loading can heat the abrasive liquid to approx.25° C., which, however, has no adverse effect on the material-removalrate.

The delivery line 14 leads into the interior of the vessel 11, where thewater 3 is accelerated to approx. 40 m/s using a nozzle 9. The nozzle 9is directed onto the surface S, which moves with the holding apparatus10 beneath it.

In a pressurized tank 13 there is air 4 which can be introduced into thewater 3 via a connection line 16. The supply of air is varied by meansof a valve 8. The higher the desired material-removal rate, the more air4 is added to the water 3. The maximum air content in the water 3 is 70%of the total volume. The pressure in the tank 13 is 250 bar. The air 4is supplied at a pressure of 14 to 50 bar. What is important in thiscontext is for the feed pressure of the air to be greater than thedelivery pressure of the water. For polishing, the air supply issuppressed or at least reduced.

The water 3 used is passed back into the tank 12 via a discharge 15. Theabrasive particles 5 are therefore in a closed circuit indicated by thearrows 6. This keeps the consumption of material as low as possible.

The air supply is calibrated on a reference workpiece (not shown). Thematerial-removal rate for the workpiece 2 can then be set as required onthe basis of this calibration.

An alternative embodiment of the apparatus 1 according to the inventionis illustrated in FIG. 2. As in the exemplary embodiment shown in FIG.1, the abrasive liquid 3 in this embodiment is likewise in a closedcircuit indicated by the arrows 6. The liquid 3 used is once againwater.

In this example, unlike in the example shown in FIG. 1, a nozzle is notused to accelerate the water 3 onto the surface S of the workpiece 2.Instead, the abrasive liquid 3 emerges through a delivery head 17. Theacceleration of the water 3 takes place in the gap 18 which is formedbetween the head 17 and the surface S. To control the width of the gap18, the holding apparatus 8 can also be displaced in a verticaldirection, as indicated by the arrows B. The smaller the surface area ofthe gap in relation to the cross-sectional area of the delivery line 14,the greater the acceleration. The ratio of the cross-sectional area ofthe delivery line to the surface area of the gap is in this example10:1.

The material-removal rate is controlled by the supply of air 4 into thewater 3 in the tank itself. Air 4 is located in the tank 12 above thewater 3 and is stirred by a propeller 19 below the water 3. Thepropeller 19 is driven by an electric motor M. The rotational speed ofthe motor M determines the quantity of air 4 supplied. Since the water 3is used in a closed circuit, the level of the water 3 is relativelyconstant. The propeller 19 is large enough in relation to the water tank12 for fluctuations in the water level to have no adverse effect on thequantity of air supplied.

FIG. 3 shows an apparatus similar to that shown in FIG. 1. In addition,however, there is an apparatus for reducing the air content in the water3 in the tank 12. The proportion of gas in the water can vary on thebasis of surrounding parameters. In certain cases, it may be desirableor necessary for the gas content to be reduced in order to achieve thedesired material-removal rate. This may be the case, for example, if thesurface is to be particularly finely polished.

In accordance with FIG. 3, ultrasound emitters 20 are arranged in thetank 12 in order to reduce the air content in the water 3. Theseemitters emit ultrasound waves 21 which displace the air inclusions inthe water 3.

Of course, other embodiments of the invention are also feasible. Theexamples mentioned here serve merely as an explanation and have norestricting effect.

1. An apparatus for removing material from a surface of a workpieceduring grinding and/or polishing of the surface by means of abrasiveparticles which are delivered by a liquid, comprising a device forsetting the gas content in the liquid.
 2. The apparatus according toclaim 1 comprising a device for admixing gas to the liquid.
 3. Theapparatus according to claim 2 wherein the gas consists of air.
 4. Theapparatus according to claim 2, wherein the device for admixing gascomprises a setting arrangement for setting the quantity of gas which isadded.
 5. The apparatus according to claim 2, wherein the device foradmixing gas to the liquid is arranged in at least one of a tank for theliquid, a pump for delivering the liquid and a delivery line for theliquid.
 6. The apparatus according to claim 2, wherein the device foradmixing gas is designed with a pressure which is greater than thedelivery pressure of the liquid.
 7. The apparatus as according to claim2, wherein the device for admixing gas is designed to admix gas to theliquid in a proportion of volume of up to 70%, measured on the basis ofthe total volume.
 8. The apparatus according to claim 6, wherein thedevice for admixing gas is designed to generate a delivery pressure ofless than 100 bar.
 9. The apparatus according to claim 6, wherein thedevice for admixing gas is designed to generate a delivery pressure ofless than 50 bar.
 10. The apparatus according to claim 1, wherein anaccelerating arrangement for accelerating the liquid is provided. 11.The apparatus according to claim 10, wherein the apparatus is designedsuch that the liquid can be accelerated to a velocity of over 20 m/s.12. The apparatus according to claim 10, wherein the accelerationarrangement has at least one nozzle.
 13. The apparatus according toclaim 10, wherein the accelerating arrangement includes an arrangementfor setting the distance between at least one outlet opening for theliquid and the surface, and is designed such that the acceleration canbe set between the at least one outlet opening and the surface as afunction of the cross section of a liquid feed opening and the crosssection of the gap between the surface and the at least one outletopening.
 14. The apparatus according to claim 1, which is designed touse abrasive particles with a mean grain diameter of less than 50 (m.15. The apparatus according to claim 14 which is designed to useabrasive particles with a mean grain diameter from 1 to 10 (m.
 16. Theapparatus according to claim 14, wherein the device is designed to admixgas in order to generate gas bubbles in the liquid with a mean diameterwhich is larger than the mean grain diameter of the abrasive particles.17. A method for controlling the material removal-rate during removal ofmaterial from a surface of a workpiece during grinding and/or polishingof surface, the method comprising the step of adding gas in a deliveryliquid for abrasive particles.
 18. The method according to claim 17wherein gas inclusions consist of air inclusions.
 19. A process forremoving material from a surface of a workpiece using abrasive particleswhich are delivered by a liquid, comprising the step of setting the gascontent in the liquid.
 20. The process according to claim 19 whereinmaterial is removed from the surface by grinding and/or polishing saidsurface.
 21. The process according to claim 19, comprising the furtherstep of adding gas to the liquid.
 22. The process according to claim 21,wherein the gas consists of air.
 23. The process according to claim 19,wherein the liquid is supplied with gas amounting to 0 to 70% of thevolume.
 24. The process according to claim 20, comprising a firstgrinding step wherein gas is admixed with the liquid for grinding thesurface, and a second polishing step wherein no gas is admixed with theliquid for polishing the surface.
 25. The process according to claim 20,wherein the surface is both ground and polished with abrasive particleswith a mean grain diameter of less than 50 (m.
 26. The process accordingto claim 25, wherein the abrasive particles have a mean grain diameterfrom 1 to 10 (m.
 27. The process according to claim 19, wherein thedelivery pressure of the liquid is kept constant at less than 100 bar,for both grinding and polishing.
 28. The process according to claim 27,wherein the delivery pressure of the liquid is kept constant at lessthan 50 bar.
 29. The process according to claim 19, wherein the liquidused is predominantly water.
 30. The process according to claim 19,wherein gas bubbles which are larger than the abrasive particles aregenerated in the liquid.